Ive done lots of UK visas for my Thai wife and mates who have Thai wives and girlfriends before.
Theres endless amounts of paperwork involved especially for a first time visit.
Main reason for rejection is no reason to return to Thailand. If she has a job, property, children and assets and you can afford to sponsor her then theres no problem.
Be prepared to submit your bank statements, work contract etc though.
You can fit an entry stamp the size of your thumb. Same for the exit.
BTW Thai immigration are very OCD when stamping your passport. I can guarantee your exit stamp will be right next to your arrival stamp when you leave. Not like some stupid countries that stamp you in on one page and out on another.